Air purification systems typically provide a purified air stream to the interiors of enclosures, such as buildings, vehicles, mobile or stationary shelters, etc. These air purification systems typically use filters, such as adsorptive filters, e.g., carbon filters, to filter, e.g., adsorb, gaseous or vaporous contaminants, such as toxic gasses or vapors, e.g., gaseous or vaporous chemical and/or biological warfare agents, from an air stream to provide a purified air stream to the enclosure. One problem with some filters is that it is often hard to determine the remaining filtering capacity of the filter (often termed the residual life of the filter), such as the remaining adsorption capacity of the filter, and thus whether the filter is still capable of filtering gaseous or vaporous contaminants from an air stream.